Author

Abstract

Skeletal muscle has the ability to increase in size (hypertrophy) after resistance is placed upon it. This hypertrophy is marked by significant upregulation of the mammalian target of rapamycin (mTOR) and its downstream targets. The upstream kinases, protein kinase B (also known as Akt) and AMP-activated protein kinase (AMPK), are two of the many regulators of the mTOR pathway. Recent studies suggest that the widely consumed neuroactive compound caffeine could potentially inhibit mTOR by acting through Akt and/or AMPK. The purpose of this thesis was to: 1) determine if caffeine can inhibit the mTOR pathway and ultimately attenuate skeletal muscle hypertrophy and 2) determine if this inhibition is through LKB1, an upstream regulator of AMPK. First, 3 month old male rats underwent unilateral tenotomy of the gastrocnemius, resulting in overloading (OVLD) of the synergistic plantaris muscle. The contralateral limb was sham-operated (SHAM) on. Rats were given ad libitum access to tap water or tap water + caffeine (1 g/L). The OVLD procedure resulted in significant hypertrophy of the plantaris which was attenuated after 1 wk of caffeine treatment. However, after two wks this effect was not observed. mTOR targets were examined in both the SHAM and OVLD plantaris muscle which showed significant upregulation with OVLD but no impact with caffeine treatment. Akt and AMPK was also assessed in the plantaris muscle which showed diminished Akt phosphorylation in 1 wk treated rats while the phosphorylation of AMPK remained relatively unaffected. Notably, caffeine caused decreased atrophy of the tenotomized gastrocnemius after 1 wk along with decreased body weight gains, food consumption, and retroperitoneal fat pad weight in both 1 and 2 wk treated rats. Second, to elucidate how caffeine could be impacting the mTOR pathway and how LKB1/AMPK might be involved, skeletal muscle specific LKB1 knockout (skmLKB1-KO) mice were subjected to high-frequency electrical stimulation (HFES) of the sciatic nerve resulting in contraction of the tibialis anterior (TA) and extensor digitorum longus (EDL) muscles against the larger gastrocnemius. All mice were given an intraperitoneal injection of saline or saline + caffeine (20 mg/kg BW at 1 g/L). HFES resulted in marked upregulation of mTOR targets in the TA/EDL of mice 0, 3, and 8 h post HFES. mTOR targets remained relatively unchanged with caffeine treatment. We also observed that these markers were consistently upregulated in our skmLKB1-KO mice with or without HFES. Our findings indicate that caffeine, at physiological concentrations, does not impact anabolic signaling. Furthermore, diminished LKB1 levels resulted in increased levels and activation of markers of protein synthesis.